1. Field of the Invention
The present invention relates generally to an apparatus and method for channel transmission in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to a channel transmitting apparatus and method in which a dedicated channel is gated if there is no transmission data for a predetermined time.
2. Description of the Related Art
A conventional CDMA mobile communication system primarily provides a voice service. However, the future CDMA mobile communication system will support the IMT-2000 standard, which can provide a high-speed data service as well as the voice service. More specifically, the IMT-2000 standard can provide a high-quality voice service, a moving picture service, an Internet search service, etc.
The CDMA mobile communication base station (BS) system operates synchronously or asynchronously. The synchronous CDMA mobile communication system is employed in U.S, while the asynchronous CDMA mobile communication system in the Europe. Thus, standardization is separately under way. As stated before, the U.S. and Europe are developing their separate standards due to the different systems. The European future mobile communication system is referred to as UMTS (Universal Mobile Telecommunication Systems) and the American future mobile communication system, CDMA-2000. The two systems use different channel structures and terms. The following description will be conducted in this context, and with the appreciation that the term “mobile communication system” used hereinafter covers both future mobile communication systems.
In the mobile communication system, a data communication service is typically characterized in that transmission of burst data alternates with long non-transmission periods. In the future mobile communication system, traffic data is transmitted on a dedicated traffic (data) channel or downlink shared channel for a data transmission duration, and the dedicated traffic channel is maintained for a predetermined time even when a base station and a mobile station have no traffic data to transmit. That is, because of limited radio resources, base station capacity and power consumption of a mobile station, the mobile communication system transmits the traffic data on the dedicated traffic channel or downlink shared channel for the data transmission duration and maintains the channel between the base station and the mobile station for a predetermined time even when there is no traffic data to transmit. This standardization minimizes a time delay due to sync reacquisition when there is traffic data to transmit.
Such a mobile communication system requires many states according to channel assignment circumstances and state information existence/non-existence in order to provide a packet data service as well as a voice service. For example, a state transition diagram for a cell connected state, a user data active substate and a control-only substate is well defined in 3GPP RAN TS S2 series S2.03.99.04.
FIG. 1A shows state transition in the cell connected state of the mobile communication system. Referring to FIG. 1A, the cell connected state includes a paging channel (PCH) state, a random access channel (RACH)/downlink shared channel (DSCH) state, an RACH/forward link access channel (FACH) state, and a dedicated channel (DCH)/DCH, DCH/DCH+DSCH, DCH/DSCH+DSCH Ctrl (Control Channel) state.
FIG. 1B shows a user data active substate and a control-only substate of the DCH/DCH, DCH/DCH+DSCH, DCH/DSCH+DSCH Ctrl state. It should be noted that the novel gated transmission device and method is applied when the data transmission channel, DCH or DSCH, is no data to transmit for a predetermined time, control-only substate.
The existing second generation CDMA mobile communication system, which mainly provides the voice service, releases a channel after completion of data transmission and connects the channel again when there is further data to transmit. However, in providing the packet data service as well as the voice service, the recommended future data transmission method has many delaying factors such as a reconnection delay, thus making it difficult to provide a high-quality service. Therefore, to provide the packet data service as well as the voice service, an improved data transmission method is required. For example, in many cases, data transmission is performed intermittently for Internet access and file downloading. Therefore, there occurs a non-transmission period between transmissions of consecutive data packets. For this period, the conventional data transmission method either releases or maintains the data channel. Releasing the data channel will require a long time in reconnecting the channel. Maintaining the data channel will cause a waste of the channel resources.
To solve such problems, a control channel (DCCH or DPCCH) is provided between the base station and the mobile station so that for the data transmission period, a control signal related to the traffic data channel signal is exchanged for power control of the data channel and for the non-transmission period, the traffic data channel is released and only the control channel is maintained. Such a state is called “control-only substate” or “control hold state”.
Even though UMTS provide a dedicated control channel, the dedicated control channel is released simultaneously with the release of the dedicated data channel. The dedicated control channel must be reconnected each time a generation of data to transmit occurs.
First, the UMTS will be described herein below.
A downlink for transmitting signals from a base station to a mobile station or an uplink for transmitting signals from the mobile station to the base station includes the following physical channels. A description of the physical channels departing from the scope of the invention will be avoided for simplicity. The physical channels include a dedicated physical control channel (DPCCH) in which pilot symbols are included for sync acquisition and channel estimation, and a dedicated physical data channel (DPDCH) or downlink shared channel (DSCH) for exchanging traffic data with a specific mobile station. The downlink DPDCH or DSCH includes traffic data. The downlink DPCCH includes, at each slot or power control group (PCG), a transport format combination indicator (TFCI) bit, which is information about the format of transmission data, transmit power control (TPC) information bit, which is a power control command, and control information such as the pilot symbols for providing a reference phase so that a receiver (the base station or the mobile station) can compensate for differences in the phase. The DPDCH and the DPCCH are time multiplexed within one PCG.
As an example, the invention will be described with reference to a case where a me length is 10 msec and each frame includes 16 PCGs, i.e., each PCG has a length of 0.625 msec. As another example, the invention will be described with reference to another case where a frame length is 10 msec, but each frame includes 15 PCGs, i.e., each PCG has a length of 0.667 msec. It will be assumed herein that the PCG (0.625 msec or 0.667 msec) has the same time period as the slot (0.625 msec or 0.667 msec). The PCG (or slot) is comprised of pilot symbol, traffic data, transmission data-related information TFCI, and power control information TPC. The values stated above are given by way of example only.
FIG. 2A shows a slot structure including the downlink DPDCH and DPCCH in the UMTS. In FIG. 2A, although the DPDCH is divided into traffic data 1 and traffic data 2, there is a case where the traffic data 1 does not exist and only the traffic data 2 exists according to the types of the traffic data. Table 1 shows the symbols constituting the downlink DPDCH/DPCCH fields, wherein the number of TFCI, TPC and pilot bits in each slot can vary according to a data rate and a spreading factor.
TABLE 1Downlink DPDCH/DPCCH FieldsChannelChannelSymbolDPDCHDPCCHBit RateRateBits/FrameBits/SlotBits/Slot(kbps)(ksps)SFDPDCHDPCCHTOTBits/SlotNdata1Ndata2NTFCINTPCNpilot16851264961601022024168512321281601002224321625616016032020280283216256128192320200822864321284801606404062402864321284481926404042422812864641120160128080145602812864649922881280806568282561283224001602560160301200282561283222722882560160221208285122561648322885120320622400216512256164704416512032054240821610245128995228810240640126496021610245128982441610240640118496821620481024420192288204801280254100802162048102442006441620480128024610088216
Unlike the downlink DPDCH and DPCCH, uplink DPDCH and DPCCH for transmitting signals from the mobile station to the base station are separated by channel separation codes.
FIG. 2B shows a slot structure including the uplink DPDCH and DPCCH in the UMTS. In FIG. 2B, the number of TFCI, FBI (FeedBack Information), TPC and pilot bits can vary according to a service option, such as the types of the traffic data or transmit antenna diversity, or a handover circumstance. The FBI is information about two antennas that the mobile station requests, when the base station uses the transmit diversity antennas. Tables 2 and 3 below show the symbols constituting the uplink DPDCH and DPCCH fields, respectively, wherein SF denotes a spreading factor.
TABLE 2Uplink DPDCH FieldsChannel Bit RateChannel SymbolBits/(kbps)Rate (ksps)SFBits/FrameSlotNdata1616256160101032321283202020646464640404012812832128080802562561625601601605125128512032032010241024410240640640
TABLE 3Uplink DPCCH FieldsChannelChannelSymbolBit RateRateBits/Bits/(kbps)(ksps)SFFrameSlotNpilotNTPCNTFCINFBI161625616010622016162561601082001616256160105221161625616010720116162561601062021616256160105122
Tables 1 to 3 show an example where there exists one DPDCH which is a traffic channel. However, there may exist second, third and fourth DPDCHs according to the service types. Further, the downlink and uplink both may include several DPDCHs.
A hardware structure of the conventional UMTS mobile communication system (base station transmitter and mobile station transmitter) will be described below with reference to FIGS. 3A and 3B. Although the base station transmitter and mobile station transmitter will be described with reference to a case where there exist three DPDCHs, the number of DPDCHs is not limited.
FIG. 3A shows a structure of a base station transmitter in the conventional UMTS. Referring to FIG. 3A, multipliers 111, 121, 131 and 132 multiply a DPCCH signal and DPDCH1, DPDCH2 and DPDCH3 signals, which have undergone channel encoding and interleaving, by gain coefficients G1, G2, G3 and G4, respectively. The gain coefficients G1, G2, G3 and G4 may have different values according to circumstances such as the service option and the handover. A multiplexer (MUX) 112 time-multiplexes the DPCCH signal and the DPDCH1 signal into the slot structure of FIG. 2A. A first serial-to-parallel (S/P) converter 113 distributes the output of the multiplexer 112 to an I channel and a Q channel. Second and third S/P converters 133 and 134 S/P-convert the DPDCH2 and DPDCH3 signals and distribute them to the I channel and the Q channel, respectively. The S/P converted I and Q channel signals are multiplied by channelization codes Cch1, Cch2 and Cch3 in multipliers 114, 122, 135, 136, 137 and 138, for spreading and channel separation. Orthogonal codes are used for the channelization codes. The I and Q channel signals multiplied by the channelization codes in the multipliers 114, 122, 135, 136, 137 and 138 are summed by first and second summers 115 and 123, respectively. That is, the I channel signals are summed by the first summer 115, and the Q channel signals are summed by the second summer 123. The output of the second summer 123 is phase shifted by 90° by a phase shifter 124. A summer 116 sums an output of the first summer 115 and an output of the phase shifter 124 to generate a complex signal I+jQ. A multiplier 117 scrambles, complex spreads, the complex signal with a PN sequence Cscramb which is uniquely assigned to each base station, and a signal separator 118 separates the scrambled signal into a real part and an imaginary part and distributes them to the I channel and the Q channel. The I and Q channel outputs of the signal separator 118 are filtered by filters 119 and 125, respectively, to generate bandwidth-limited signals. The output signals of the filters 119 and 125 are multiplied by carriers cos{2πfct} and sin{2πfct} in multipliers 120 and 126, respectively, to frequency the signals to a radio frequency (RF) band. A summer 127 sums the frequency-shifted shifted I and Q channel signals.
FIG. 3B shows a structure of a mobile station transmitter in the conventional UMTS. Referring to FIG. 3B, multipliers 211, 221, 223 and 225 multiply a DPCCH signal and DPDCH1, DPDCH2 and DPDCH3 signals, which have undergone channel encoding and interleaving, by channelization codes Cch1, Cch2, Cch3 and Cch4, respectively, for spreading and channel separation. Orthogonal codes are used for the channelization codes. The output signals of the multipliers 211, 221, 223 and 225 are multiplied by gain coefficients G1, G2, G3 and G4 in multipliers 212, 222, 224 and 226, respectively. The gain coefficients G1, G2, G3 and G4 may have different values. The outputs of the multipliers 212 and 222 are summed by a first summer 213 and output as an I channel signal, and the outputs of the multipliers 224 and 226 are summed by a second summer 227 and output as a Q channel signal. The Q channel signal output from the second summer 227 is phase shifted by 90° in a phase shifter 228. A summer 214 sums the output of the first summer 213 and the output of the phase shifter 228 to generate a complex signal I+jQ. A multiplier 215 scrambles, complex spreads, the complex signal with a PN sequence Cscramb which is uniquely assigned to each base station, and a signal separator 229 separates the scrambled signal into a real part and an imaginary part and distributes them to the I channel and the Q channel. The I and Q channel outputs of the signal separator 229 are filtered by filters 216 and 230, respectively, to generate bandwidth-limited signals. The output signals of the filters 216 and 230 are multiplied by carriers cos{2πfct} and sin{2πfct} in multipliers 217 and 231, respectively, to frequency shift the signals to a radio frequency (RF) band. A summer 218 sums the frequency-shifted I and Q channel signals.
FIG. 5A illustrates transmission of a downlink DPCCH and an uplink DPCCH when transmission of an uplink DPDCH is discontinued, that is, traffic data to transmit does not exist for a predetermined time in the conventional UMTS. The state wherein the traffic data does not exist for a predetermined time is called “control-only substate”.
FIG. 5B illustrates transmission of the downlink DPCCH and the uplink DPCCH when transmission of a downlink DPDCH is discontinued, that is, traffic data to transmit does not exist for a predetermined time in the conventional UMTS.
As shown in FIGS. 5A and 5B, the mobile station continuously transmits the uplink DPCCH in the absence of traffic data in order to avoid sync reacquisition from the base station. Meanwhile, if there exists no traffic data to transmit for a long time in the continuous uplink DPCCH transmission state, the base station and the mobile station transit to an RRC (Radio Resource Control) connection released state (not shown). Although the transmission of the uplink DPDCH is discontinued, the mobile station continuously transmits DPCCH signal on the uplink DPCCH. The resulting increase of uplink interference reduces the capacity of the uplink.
Despite the advantage of avoidance of sync reacquisition from the base station, the continuous uplink DPCCH transmission in a control-only substate in the conventional UMTS increases the uplink interference and reduces the uplink capacity. Furthermore, since downlink PCBs are continuously transmitted on the downlink DPCCH, downlink interference increases and downlink capacity decreases. Therefore, it is necessary to minimize, in case of release the channels, both the time required for sync reacquisition from the base station and, in case of continuous transmission of DPCCH signals, the increase of uplink and downlink interference.
The second future mobile communication system, the CDMA-2000 system, will now be described.
As stated above, a CDMA-2000 system provides a dedicated control channel (DCCH) to prevent channel consumption caused by maintaining a channel even when there is no traffic data to transmit. That is, a control signal related to a dedicated data channel (Fundamental or Supplemental Channel ) is exchanged between a base station and a mobile station for a data transmission period. For a non-data transmission period, the dedicated data channel is released and only the dedicated control channel is maintained. Consequently, channel consumption is prevented and the dedicated data channel can be rapidly set up using the DCCH upon generation of transmission data. This state is called “control hold state” in CDMA-2000. The control hold state is divided into two substates: normal substate and slotted substate. In the normal substate, there is no data to transmit on a communication channel and only a control signal is communicated on a DCCH. In the slotted substate, a control signal is not even communicated due to no communication of packet data for a long time in the normal substate. In the transition from the slotted substate to the normal substate, resynchronization is required between the base station and the mobile station because no control signals have been exchanged between them. A CDMA-2000 system can be so configured that only the normal substate is set without the slotted substate.
The structure of a conventional CDMA-2000 mobile communication system for transmitting a signal in a control hold state will be described on the assumption that a frame is 20 ms in duration, one frame includes 16 PCGs (i.e., one PCG is 1.25 msec in duration), and a DCCH frame is 5 ms or 20 ms in duration.
FIG. 3C is a block diagram of a base station transmitter in a conventional CDMA-2000 mobile communication system. A forward link on which a base station transmits signals to a mobile station is comprised of the following channels: a pilot channel, which provides a basis for sync acquisition and channel estimation, an F-CCCH (Forward Common Control Channel) for transmitting a control message to all mobile stations within the cell of the base station, an F-DCCH (Forward Dedicated Control Channel) for transmitting a control message to a particular mobile station, and an F-DTCH (Forward Dedicated Traffic Channel) for transmitting traffic data to a particular mobile station. The F-DCCH includes a sharable F-DCCH for transmitting a control message to the particular mobile station in time division. The F-DTCH includes an F-FCH (Forward Fundamental Channel) and an F-SCH (Forward Supplemental Channel).
In FIG. 3C, demultiplexers (DEMUXes) or SPCs (Serial-to-Parallel Converters) 120, 122, 124, and 126 separate channel encoded and interleaved data into I channel and Q channel data. Mixers 110 and 130 to 137 multiply the separated data by corresponding orthogonal codes (e.g., Walsh codes W) for spreading and channelization. To express the outputs of the mixers 110 and 130 to 137 as relative sizes to that of a forward pilot channel, they pass through amplifiers 140 to 147. Summers 150 and 152 sum the outputs of the amplifiers 140 and 141 to 147 by I channels and Q channels. A complex spreader 160 scrambles the outputs of the summers 150 and 152 by a PN sequence assigned to the base station. The complex spread signal from the complex spreader 160 is filtered by filters 170 and 171 to generate I and Q channel signals in limited bandwidths. Amplifiers 172 and 173 amplify the outputs of the filters 170 and 171 to a signal strength suitable for transmission. Mixers 174 and 175 transits the outputs of the amplifiers 172 and 173 to an RF band by multiplying the outputs of the amplifiers 172 and 173 by carriers. A summer 180 sums the I channel and Q channel signals.
FIG. 3D is a block diagram of a mobile station transmitter in the conventional CDMA-2000 mobile communication system. A reverse link is comprised of a pilot/PCB channel on which a pilot signal for sync acquisition and channel estimation and forward PCBs for forward power control are multiplexed, an R-DCCH (Reverse Dedicated Control Channel) for transmitting a control message to the serving base station of the mobile station, and an R-DTCH (Reverse Dedicated Traffic Channel) for transmitting traffic data to the base station. The R-DTCH includes an R-FCH (Reverse Fundamental Channel) and an R-SCH (Reverse Supplemental Channel).
In FIG. 3D, a multiplexer (MUX) 210 multiplexes a reverse pilot channel and forward PCBs. Mixers 220, 230, 240, 250 and 260 multiply the reverse channel which was channel-encoded and interleaved by orthogonal codes mutually orthogonal among channels, for channelization and spreading. To express the outputs of the mixers 220, 240, 250 and 260 in relative sizes to that of the output of mixer 230 for the reverse pilot/PCB, they pass through amplifiers 222, 242, 252, and 262. Summers 224 and 254 sum the outputs of the multiplier 230 and the amplifiers 222, 242, 252, and 262 by I channels and Q channels. The complex spreader 160 scrambles the outputs of the summers 224 and 254 by a PN sequence assigned to the mobile station. The complex spread signal from the complex spreader 160 is filtered by the filters 170 and 171 to generate I and Q channel signals in limited bandwidths. The amplifiers 172 and 173 amplify the outputs of the filters 170 and 171 to a signal strength suitable for transmission. The mixers 174 and 175 transform the outputs of the amplifiers 172 and 173 to an RF band by multiplying the outputs of the amplifiers 172 and 173 by carriers. The summer 180 sums the I channel and Q channel signals.
The structure of signals communicated between a base station and a mobile station in the conventional CDMA-2000 system will now be described herein below.
Reference numeral 300 in FIG. 5C indicates a reverse pilot/PCB channel signal when an R-DCCH signal is continuous transmitted in a conventional control hold state/normal substate. A mobile station transmits the reverse pilot/PCB channel continuously in the control hold state/normal substate to avoid sync reacquisition from a base station. The resulting increase in reverse link interference reduces the capacity of the reverse link.
Reference numeral 400 in FIG. 13A indicates the generation position of an R-DCCH upon generation of a reverse dedicated MAC (Medium Access Control) channel (dmch) data in the conventional control hold state/normal substate. The R-DCCH can be transmitted within 5 msec at longest after the dmch is generated. Here, the R-DCCH can be disposed at a position being a multiple of 5 msec position only. Due to the limited positions, the base station determines whether the R-DCCH exists or not at four positions in one frame. A 2.5 msec delay on the average occurs until the R-DCCH is transmitted after generation of the dmch.
As described above, the continuous transmission of a reverse pilot/PCB channel in a control hold state/normal substate in the conventional CDMA-2000 system is advantageous in that it is possible to avoid sync reacquisition from a base station. However, the resulting increase of reverse link interference reduces the capacity of the reverse link. Further, continuous transmission of reverse PCBs on a forward link also increases forward link interference and reduces forward link capacity. Accordingly, it is necessary, in case of no transmission of PCBs, to minimize time required for sync reacquisition from the base station, in case of continuous transmission, to minimize the reverse link interference increased by the transmission of the reverse pilot/PCB channel, and the forward link interference increased by the transmission of the reverse PCBs on the forward link.